As Kant points out in a famous letter to his student Marcus Herz (10:
129–130), the leading question of his Critical philosophy in
general and of (what eventually would become) the Critique of Pure
Reason in particular is: “what is the ground of the
reference of that in us which we call ‘representation’
(‘Vorstellung’) to the object?” In other
words: how are objectively valid (and in particular, a priori) mental
representations possible? This is the fundamental topic of
Kant’s “theory of cognition”
(Erkenntnistheorie). The theory of cognition in Kant’s
sense however should not be confused with epistemology or theory of
knowledge in the contemporary sense, the special theory of justified
true belief (or justified true belief plus X, to allow for
the Gettier problem about cognitive-semantic luck) with special
reference to skepticism. Thus the first Critique is a
treatise in cognitive semantics, not a treatise in
epistemology. But within his overarching cognitive-semantic framework,
Kant also works out an extremely interesting and important
epistemology of judgment.

In this connection, the contrast between Kant’s theory of
judgment and Frege’s theory of judgment is especially relevant.
For Frege, propositional contents or thoughts, composed of cognitively
significant, non-spatiotemporally existing “senses”
(Sinne) that uniquely determine worldly complexes consisting
of objects and “concepts,”, i.e., not Kantian concepts,
but instead unsaturated n-place functions from objects to truth-values
(roughly, real properties and real relations), are somehow
“grasped”; and then judgment consists in the rational
cognitive subject’s advance from the somehow-grasped thought to
the truth-value. Hence for Frege, since truth-values occur at the
level of the reference or Bedeutung of propositions,
and not at the level of their sense, the rational cognitive
subject engages with truth or falsity only in the context of
assertoric propositional attitudes. But according to Kant, for whom
judgment is the cognitive-semantic core of all rational human
activity, and for whom episodes of judgment-making are essentially
proposition-generating intentional actions
(Handlungen)(A69/94), any propositional attitude whatsoever
is an instance of “taking-for-true” (das
Fürwahrhalten) (A820/B848)(9: 66), and thereby constitutes a
determinate way in which the rational cognitive agent directly engages
with the truth-value of the judgment.

Taking-for-true, in turn, has three basic kinds: (i)
“opining” (Meinen), (ii) “scientific
knowing” (Wissen), and (iii)“believing”
(Glauben) (A820–831/B848–859). Opining is an
epistemic propositional attitude that falls short of
“conviction” (Überzeugung), i.e., objective
sufficiency for the rational/judging subject, and also falls short of
“persuasion” (Überredung), i.e., mere
subjective sufficiency for the rational/judging subject. Hence opining
includes such subjectively and objectively unconvinced attitudes as
entertaining a proposition, fiction, supposition, etc.

Epistemic believing, by contrast, includes subjective sufficiency or
persuasion for the rational/judging subject, but also, on its own,
falls short of conviction, which includes both subjective sufficiency
or persuasion and also objective sufficiency, which itself,
in turn, necessarily includes truth in such a way as to rule out any
sort of accidental connection between epistemic believing and truth,
i.e., cognitive-semantic luck, and for that reason is also called
“certainty” (Gewisshheit). Finally, then,
scientific knowing is perfected epistemic believing that has achieved
conviction, i.e., objective sufficiency or certainty.

Unlike many 20th and 21st century epistemologists, Kant holds that a
posteriori scientific knowing or empirical certainty about contingent
truths is not only really possible, but also realized, in natural
science and everyday experience, and also that a priori scientific
knowing or non-empirical certainty about necessary truths is really
possible and realized in the exact sciences and philosophy. A priori
scientific knowing or non-empirical certainty can be either (i)
non-inferential and possessed of an intrinsically compelling cognitive
phenomenology, in which case it is rational “insight”
(Einsicht), i.e., what Descartes would have called
“clear and distinct rational intuition,” or else (ii)
inferential, i.e., based on logical demonstration or proof
(A783–794/B810–822), although even in this case its status
as scientific knowing is ultimately grounded on premises guaranteed by
rational insight (Hanna 2006, ch. 7). Because Kant distinguishes
carefully between (i) the intrinsically compelling phenomenology that
is characteristic of rational insight, and (ii) the objective
sufficiency of a priori epistemic belief and its corresponding
non-accidental or luck-avoiding connection with necessary truth, it
follows that factor (i) can obtain even if factor (ii) does not
obtain. Hence importantly unlike Descartes and other classical
infallibilist Rationalists, but also importantly like recent
“moderate” Rationalists, Kant is a fallibilist
about rational insight, a.k.a. rational intuition (see, e.g., BonJour
1998).

As several recent Kant-interpreters have correctly pointed out and
effectively elaborated, however, believing or Glauben for
Kant not only occurs in an epistemic mode, but can also occur in
several different non-epistemic modes; hence epistemic believing, as
complex and important as it is, must also be placed in the larger
context of a Kantian ethics of belief (Stevenson 2003, Chignell 2007a,
Chignell 2007b, Kain 2010, Pasternack 2011, Pasternack 2014). More
precisely, for Kant believing or Glauben divides into four
distinct doxic sub-kinds: (a) epistemic (or
scientific-knowing-oriented) belief, (b) “doctrinal” (or
speculative theoretical) belief, (c) “pragmatic” (or
instrumental) belief, and finally (d) “moral” (or ethical)
belief, the last of which, in turn, might be most illuminatingly
captured in English as ‘believing-in’, although the word
Glauben, in this connection, is usually misleadingly
translated as ‘faith’.This translation is misleading
precisely because ‘faith’ in English can, notoriously,
refer to non-rational or even irrational mental states of various
kinds; but what Kant means by the notion of moral belief or
believing-in is an epistemically objectively insufficient
propositional attitude that nevertheless provides full and sufficient
practical justification, in relation to a reasons-providing
system of moral principles grounded in the Categorical Imperative, for
choosing and acting as if, per impossibile, one were also in
a position to have epistemic belief and scientific knowledge with
respect to some truth-valueless proposition about noumenal objects or
noumenal subjects. On the contrary, the closest a rational cognitive
subject can get to anything epistemic or scientific about such a
proposition is to have “doctrinal” or purely speculative
theoretical belief with respect to it, and, as Kant notes,
“there is something unstable about merely doctrinal belief
[because] one is often put off from it by difficulties that come up in
speculation” (A827/B855), i.e., rationally unavoidable
dialectical fallacies and contradictions. He then also immediately
points out that “it is entirely otherwise in the case of
moral belief.... [f]or there it is absolutely
necessary that something must happen, namely that I fulfill the moral
law in all points” (A828/B856). The most important kinds of
moral belief or believing-in are also what Kant calls
“postulates of pure practical reason” (5: 122–134),
which most notably include the immortality of the soul and the
existence of God. In Kant’s ethics of belief, the soul’s
immortality and God’s existence (and also, just as
significantly, God’s non-existence) are propositional targets
that are impossible-to-believe-epistemically and also
impossible-to-know-scientifically, yet at the same time they specify
certain morally obligatory ways of living one’s life as a
rational human agent.

Therefore, when Kant famously writes in the B Preface to the
Critique of Pure Reason that “I had to deny
Wissen in order to make room for
Glauben” (Bxxx), contrary to what is
often taken to be his meaning, what he is actually saying is
that he had to restrict the scope of epistemic belief and scientific
knowing, by means of his Critical epistemology and transcendental
idealist metaphysics, in order to make room for fully and sufficiently
practically justified moral belief or believing-in. This is also known
as Kant’s doctrine of the primacy of the practical. In
this way, moral belief philosophically trumps epistemic belief
(including the exact sciences) and also doctrinal belief (including
any kind of speculative metaphysics). But at the same time, the
rational ground of this philosophically basic propositional attitude
is the judgment, and in that sense Kant’s theory of
judgment also grounds his meta-philosophy.